1. Field of the Invention
This invention relates to methods and apparatus for determining the actual decibel attenuation across an optical fiber splice. This invention enables the craftsman splicing the optical fibers in the field to determine at the time the splice is made whether the splice has acceptable attenuation for actual use.
2. Introduction to the Invention
The most widely used optical fibers comprise a glass core, a glass cladding which surrounds the core and a buffer which surrounds the cladding and is composed of a relatively soft polymeric material. Where reference is made in this specification to an "optical fiber," it is to be understood that the fiber referred to is one comprising a glass core, a glass or plastic cladding and a buffer. The buffer protects the cladding and the core and allows the fiber to be bent into and maintained in a bend of substantially smaller bend radius than would otherwise be possible. The buffer may be surrounded by a jacket. Optical signals are conventionally fed into optical fibers by directing them axially at the exposed fiber end. It is also known to feed an optical signal into a fiber through the side of the cladding or buffer. The buffer of the fiber is retained. The light is launched through the buffer and cladding and then into the core of the fiber.
It is often desirable to bring two optical fibers into precise axial alignment, so that a signal can be passed from one fiber to the other with minimum of loss. It is generally desirable to join the fibers together into a permanent joint, called a splice. An optical fiber may be spliced to another optical fiber either by fusing the fibers together or by adhering them together with an index-matching adhesive, i.e., an adhesive whose refractive index is chosen so that signal loss is minimized. Alternatively, the splice can be made reversable by joining the fibers together with a thermoplastic index-matching material which can be heated to allow the fibers to be inserted and aligned within the material, cooled to solidify, and form the splice then can be heated to remove the fibers again. The index-matching adhesives referred to above are generally curable adhesives which are cured chemically or with ultra-violet light. This invention can be used in all these methods of splicing optical fibers as well as in mechanical methods of splicing optical fibers.
The known methods for splicing optical fibers suffer from various disadvantages. In the field, optical fibers are frequently furnished in two kilometer lengths which are then spliced together to form a continual optical fiber of the required length. Using end launch light to optimize the splice is impractical because the light must be launched in the end of the first fiber at least two kilometers away from the splice and the light must be read at the other end of the second fiber at least two kilometers in the opposite direction. The person performing the alignment of the fibers and effecting the splice must be in communication with and respond to directions from the other persons at the ends of the first and second fibers. An OTDR (optical time domain reflectometer) can also be used to determine a splice loss, but OTDR requires one person at the light launch/light read site at the end of the fiber and another person to make the splice and requires communication between these persons usually at least one or two kilometers apart.
A more practical splicing method has been developed and is described in copending application U.S. Ser. No. 437,053 filed Oct. 27, 1982, which is a continuation-in-part of copending application U.S. Ser. No. 370,321 filed Apr. 21, 1982, (now abandoned) which is a continuation-in-part of U.S. Ser. No. 258,079 filed Apr. 27, 1982, now abandoned. The disclosures and drawings of these copending cases are incorporated herein in their entirety by reference. The method and apparatus of the present invention is particularly applicable to the disclosure in said copending cases. In said copending cases, light is side launched into the first fiber near the splice and detected through a side exit from the second fiber, also near the splice. The person performing the splice controls the light launch, reads the detector and can manually or automatically optimize the amount of light passing through the splice before the adhesive is cured. This method is very effective for obtaining the best splice for a given pair of fibers and given adhesive. However, method of said copending cases does not provide a way for determining what the actual or absolute attenuation or loss across the final splice will be.
It is important to be able to determine the actual or absolute loss of each splice at the time it is made. Until now, the determination of the actual loss across a splice was through the end launch and end read method or through use of an OTDR. These methods are impractical in many applications because it involves detecting a signal at least a kilometer from the splice, comparing the signal to the light launched into the fiber at least one kilometer in the other direction from the splice, and communicating the results to the person performing the splice. If the splice has excessive loss, it must be removed and the splice re-done.